This invention relates to protective coating systems for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a process for forming an improved aluminide bond coat of a thermal barrier coating system, such as of the type used to protect gas turbine engine components.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, such alloys alone are often susceptible to damage by oxidation and hot corrosion attack and may not retain adequate mechanical properties. For this reason, these components are often protected by an environmental and/or thermal-insulating coating, the latter of which is termed a thermal barrier coating (TBC) system. Ceramic materials and particularly yttria-stabilized zirconia (YSZ) are widely used as a thermal barrier coating (TBC), or topcoat, of TBC systems used on gas turbine engine components. TBC employed in the highest temperature regions of gas turbine engines is typically deposited by electron beam physical vapor deposition (EBPVD) techniques that yield a columnar grain structure that is able to expand and contract without causing damaging stresses that lead to spallation.
To be effective, TBC systems must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between ceramic topcoat materials and the superalloy substrates they protect. To promote adhesion and extend the service life of a TBC system, an oxidation-resistant bond coat is often employed. Bond coats are typically in the form of overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), or diffusion aluminide coatings. A notable example of a diffusion aluminide bond coat contains platinum aluminide (Ni(Pt)Al) intermetallic. When a bond coat is applied, a zone of chemical interaction occurs within the surface of the superalloy substrate beneath the coating. This zone is typically referred to as a diffusion zone (DZ), and results from the interdiffusion between the coating and substrate. The diffusion zone beneath an overlay bond coat is typically much thinner than the diffusion zone beneath a diffusion bond coat.
During the deposition of the ceramic TBC and subsequent exposures to high temperatures, such as during engine operation, bond coats of the type described above form a tightly adherent alumina (Al2O3) layer or scale that adheres the TBC to the bond coat. The service life of a TBC system is typically limited by a spallation event brought on by thermal fatigue. Spallation of TBC deposited on MCrAlX bond coats generally occurs within the TBC near the TBC-to-alumina interface, while TBC deposited on diffusion aluminide bond coats typically spall at the alumina-to-bond coat interface or within the alumina layer itself. As a result, the alumina-to-bond coat interface is particularly critical for TBC systems that employ diffusion aluminide bond coats because spallation events often initiate at this interface.
In view of the above, it can be appreciated that bond coats have a considerable effect on the spallation resistance of the TBC, and therefore TBC system life. Consequently, improvements in TBC life have been continuously sought, often through modifications to the chemistries of the bond coat. The effect of the surface finish of MCrAlY bond coats has also been investigated, as evidenced by U.S. Pat. No. 4,414,249 to Ulion et al. The results of this investigation showed that the service life of a columnar TBC can be improved by polishing an MCrAlY bond coat before depositing the TBC. The benefit of improving the surface finish of an MCrAlY bond coat is believed to be that a smoother alumina layer grows, which in turn provides a more uniform surface upon which the columnar TBC is deposited. The initial portion of a columnar TBC consists of many small grains that appear to grow in a competitive fashion, by which more favorably oriented grains eventually dominate less favorably oriented grains. By polishing an MCrAlY bond coat, it is believed that Ulion et al. reduced the number of nucleated grains, thereby reducing growth competition and improving the quality of the TBC adjacent the alumina scale, i.e., in the very region that TBC spallation tends to occur on an MCrAlY bond coat.
As noted above, TBC spallation initiates by a different mechanism on diffusion aluminide bond coats, and primarily along the alumina-bond coat interface. As a result, the toughness of the alumina and the alumina-bond coat interface are most important to TBC deposited on a diffusion aluminide bond coat. From this perspective, polishing a diffusion aluminide bond coat would be expected to reduce TBC life, since sufficient surface roughness of the bond coat would be desired to promote adhesion of the alumina to the bond coat, and to provide a tortuous path that inhibits crack propagation through the alumina and alumina-bond coat interface. As a result, conventional practice has been to grit blast the surface of a diffusion aluminide bond coat to increase its roughness to about 50 microinches (about 1.25 micrometers) Ra or more before depositing the TBC.
The present invention generally provides a method for improving the thermal fatigue life of a thermal barrier coating (TBC) deposited on an aluminide bond coat through a process by which the surface of the aluminide bond coat is modified to eliminate or at least reduce oxidation and oxidation-induced convolutions at the alumina-bond coat interface, as explained more fully below. The bond coat can be a single-phase [(Ni,Pt)Al] or two-phase [PtAl2+(Ni,Pt)Al] diffusion aluminide, though it is believed that overlay aluminide bond coats can also benefit from the teachings of this invention. The invention is particularly directed to aluminide bond coats deposited by methods that produce a generally columnar grain structure, in which grains extend through the additive layer of the bond coat, i.e., from the diffusion zone beneath the additive layer to the bond coat surface, such that grain boundaries are exposed at the bond coat surface. Two widely-used methods that produce bond coats of this character are vapor phase aluminizing (VPA) and chemical vapor deposition (CVD). The surface of a bond coat having columnar grains is characterized by surface irregularities, termed grain boundary ridges, that correspond to locations where grain boundaries meet the bond coat surface.
According to the invention, the spallation resistance of a thermal barrier coating on a diffusion aluminide bond coat is improved by modifying the surface of the bond coat, by which a sufficient amount of material is removed from the grain boundary ridges and other surface peaks to flatten the bond coat surface, i.e., the ridges and peaks are replaced with flattened surfaces that are nearly parallel to the diffusion zone of the bond coat. The invention is particularly directed to flattening the grain boundary ridges formed where the grain boundaries intersect the bond coat surface. By removing these surface irregularities, it is believed that a more stable bond coat surface is created where the critical alumina-bond coat interface will exist following a thermal treatment, such as TBC deposition. Various techniques are believed to be suitable for modifying a diffusion aluminide bond coat surface for purposes of this invention, including controlled polishing, vibrolapping and tumbling techniques.
According to this invention, the original columnar grains of an as-deposited aluminide bond coat were found to be prone to accelerated oxidation at their grain boundaries, with oxidation initiating at the bond coat surface. Unexpectedly, flattened grain boundaries were shown to be much less prone to accelerated oxidation than the original grain boundaries. Surface modification in accordance with this invention also appears to significantly inhibit thermal grooving (the formation of valleys between adjacent grains), and thermal creep that has been determined to initiate and/or rapidly progress at grain boundaries exposed at the bond coat surface. A lower oxidation rate at the grain boundaries may eliminate a cause for the creation of stress concentration sites for enhanced localized creep and oxide crack initiation at the bond coat surface, which are believed to cause the alumina layer to convolute and fracture. Another possibility is that the modified bond coat grain configuration exhibits more stable surface tension conditions, which slow the thermal grooving effect. By eliminating or at least inhibiting the formation of sites where deformation of the alumina layer occurs, and thus where a fracture ultimately initiates and develops with thermal cycling, the spallation life of the TBC adhered by the bond coat is significantly increased.
Also observed with this invention is the development during subsequent thermal treatments of new triangular-shaped grains where grain boundaries originally intersected the surface of the bond coat. These triangular-shaped grains have not been observed to exhibit oxide convolutions found with unmodified diffusion bond coats and associated with spallation events.
Other objects and advantages of this invention will be better appreciated from the following detailed description.